Gastrointestinal Tract




CHAPTER 16 Gastrointestinal Tract



It is unrealistic to think that we can thoroughly address every aspect of gastrointestinal (GI) anatomy and pathophysiology in a textbook of this scope and purpose. Consequently, we focus our discussion on diseases and quandaries likely to be encountered in a typical clinical practice. Our goal is to lead the reader from an imaging finding toward a specific diagnosis or manageable list of likely diagnoses. After a brief discussion of alimentary tract anatomy and imaging modalities, this chapter focuses on diagnostic possibilities for common imaging findings in each section of the alimentary tract, including wall thickening, luminal narrowing, dilatation, ulceration, and masses. This chapter does not always specify a modality when discussing the findings of specific abnormalities. This is to encourage thinking across all modalities, because most radiologists who interpret abdominal studies do not practice in a specific modality, and because many diseases of the alimentary tract manifest on more than one type of imaging study. Furthermore, advances in technology have allowed cross-sectional modalities such as computed tomography (CT) and magnetic resonance imaging (MRI) to create images remarkably similar to fluoroscopic studies (Fig. 16-1).




ANATOMY OF THE GASTROINTESTINAL TRACT



Alimentary Tube




Duodenum


The duodenum is divided into four parts. The first part, the duodenal bulb, lies in close proximity to the gallbladder. This close association explains how a cholecystoduodenal fistula can result from the erosion of a gallstone into the duodenum or perforation of a duodenal ulcer into the gallbladder. The duodenal bulb is the most mobile of the duodenal segments, because it is almost completely invested in peritoneum (the exception being its posterior surface near the gallbladder neck and inferior vena cava). The gastroduodenal artery courses posterior to the first portion of the duodenum, explaining the greater rate of GI hemorrhage associated with ulcers of the posterior duodenal wall.


The second part of the duodenum (referred to as the descending portion) borders the right lateral margin of the pancreatic head, and receives the common bile and pancreatic ducts at the major papilla. Therefore, abnormalities of the distal common bile duct, ampulla, or pancreatic head can involve the descending duodenum. The location of the major papilla is suggested on double-contrast barium examinations by the presence of several longitudinal or oblique folds. When there is a minor papilla as well, it is typically about 1 cm proximal and slightly anterior to the major papilla. The posterior surface of the descending duodenum is in continuity with the anterior pararenal space. The transverse colon passes anterior to the middle third of the descending duodenum. They are linked by the duodenocolic ligament, which serves as another bidirectional conduit for inflammation or tumor.


The third portion of the duodenum (the transverse portion) crosses midline from right to left, immediately ventral to the anterior wall of the aorta and dorsal to the origin of the superior mesenteric artery (SMA). This relation explains why aortoenteric fistulas typically involve the third portion of the duodenum. The transverse duodenum is also bordered superiorly by the pancreatic head and, like the second portion of the duodenum, occupies the anterior pararenal space. Contiguity with the pancreas permits frequent involvement of the duodenum with pancreatic cancers and pancreatitis.


The fourth portion of the duodenum becomes largely invested in peritoneum before joining the jejunum. A suspensory ligament (of Treitz) consisting of muscle fibers and fibrous tissue extends from the region of the celiac axis and left diaphragmatic crus to the duodenojejunal junction.


Mucus-secreting Brunner glands are found in the duodenum and most proximal jejunum but are largest and most numerous near the pylorus.




Colon and Rectum


The colon extends from the cecum to the rectum and consists of ascending, transverse, descending, and sigmoid segments. The cecum is a blind ending pouch extending caudal to the ileocecal valve, typically in the right lower quadrant. The cecum is variably covered in peritoneum and potentially mobile. The appendiceal lumen communicates with that of the cecum. The appendix is notoriously variable in length (up to 20 cm) and location, and can extend far cephalad into the right upper quadrant or caudally into the inguinal canal. It can cross the midline or curl behind the cecum (retrocecal appendix). The ileocecal valve consists of mucosal membrane and circular muscle fibers. It variably allows passage of contrast material on retrograde fluoroscopic studies and is often quite fatty on CT and MRI studies.


The ascending and descending colon and rectum are extraperitoneal, whereas the transverse and sigmoid colon are intraperitoneal, invested in, and suspended by the transverse and sigmoid mesocolons, respectively. The longitudinal muscle of the colon wall is arranged in three parallel bands (taeniae coli), and the contour of the colon is marked by haustrations. Fatty appendages (appendices epiploicae) attach to the serosal surface of the colon. These are normally inconspicuous unless outlined by ascites (Fig. 16-2). On double-contrast barium examination of the colon, one can sometimes see the fine innominate lines (underdistended colon), transverse folds (contracted colon), and lymph follicles that may have central umbilication (young patients) (Fig. 16-3). Lymph follicles can be associated with lymphoma, carcinoma, and inflammatory processes, and when conspicuous in older patients, should prompt a search for an associated abnormality.





Bowel Wall


The basic layers of the GI tract wall are illustrated in Figure 16-4. The mucosa is best depicted with double-contrast barium studies, although one can only infer information about the bowel wall with fluoroscopy. Using ultrasound (US) under ideal conditions, one can distinctly identify as many as five bowel wall layers (Fig. 16-5). In many circumstances, however, only a central echogenic area (mucosa and submucosa) surrounded by a hypoechoic rim (muscularis propria) can be distinguished.




CT and MRI are usually incapable of resolving the bowel wall to the same extent as US. With contrastenhanced CT, the bowel wall displays stratification during the arterial phase with enhancing mucosa, hypoenhancing submucosa, and moderately enhancing muscularis propria all visible. Visualization of bowel wall layers depends on rate of injection and phase of imaging, and is better appreciated with intraluminal water rather than opaque oral contrast media. The presence of submucosal edema or hemorrhage contributes to a stratified appearance of the bowel.


The normal gastric wall is approximately 5 mm thick as measured between rugal folds in a distended state. The valvulae conniventes of the small bowel are normally less than 2 to 3 mm thick in distended loops, and the normal small bowel wall is only 1 to 2 mm thick when the lumen is sufficiently distended (diameter ≥ 2 cm). The normal colon wall thickness also measures only a few millimeters when adequately distended. Once considered an indicator of inflammatory bowel disease (Fig. 16-6), intramural fat can occur normally in the small intestine and colon of obese patients. This fatty layer becomes less conspicuous with luminal distention.



With any imaging modality, the normal bowel wall can appear thickened if the bowel lumen is not sufficiently distended. It is important to evaluate segments that are distended with gas, fluid, or enteric contrast material to decide whether apparent bowel wall thickening is real, keeping in mind that lumen narrowing can result from mural thickening. Coexisting findings such as adjacent edema, abnormal wall enhancement, or vascular engorgement on CT or MRI examinations can improve diagnostic confidence. Spurious wall thickening tends to be transient, and real-time imaging modalities such as fluoroscopy and US can determine whether an area of questionable wall thickening persists. With many CT and MRI protocols, multiphase or delayed acquisitions are performed, and comparing different phases can help confirm whether an area of bowel wall thickening is fixed or transient. Newer MRI protocols designed to assess the bowel often include rapid imaging sequences such as steady-state free precession that also allow the peristaltic activity of the bowel to be observed.



Bowel Rotation


Normal orientation of the intestinal tract occurs only after the embryologic gut completes a complex series of maneuvers. Failure of any one of these steps results in a variety of congenital abnormalities of the bowel. These developmental maneuvers include elongation of the gut and suspending mesentery, herniation of the gut into the base of the umbilical cord, rotation, and return of the developing gut to the abdominal cavity. It is important to recognize when the process of gut rotation and fixation has gone awry. Box 16-1 lists imaging findings that suggest a gut rotational abnormality. The clinical significance of abnormal rotation and fixation of the bowel is the increased risk for intestinal volvulus, especially in neonates and infants.





IMAGING MODALITIES



Radiography (Plain Films)


Radiographs are still widely performed for the evaluation of abdominal complaints. Supine portable radiographs can confirm enteric tube placement, identify calcifications and foreign bodies, suggest a mass or organomegaly, and evaluate for obstruction. The combination of supine and upright radiographs is generally preferred to supine images alone for detection of extraluminal (i.e., intraperitoneal, intramural, or intravascular) gas and for assessment of the bowel gas pattern. Wall thickening can be inferred on abdominal radiography, although the bowel wall is not directly visible in the absence of peritoneal gas (Fig. 16-8). Bowel obstruction causes dilated gas-filled bowel loops proximal to the obstruction, with a lack of gas or bowel contents distal to the obstruction. Dilated fluid-filled bowel loops can escape radiographic detection, although obstruction can be suspected when bubbles of gas collect between mucosal folds (“string-of-pearls sign”).



Pneumatosis intestinalis can be a subtle finding on radiographs, imparting a bubbly appearance to the bowel that is often mistaken for stool (Fig. 16-9). The presence of pneumatosis should prompt a search for portal venous gas, best seen overlying the periphery of the liver, often on the left of a supine patient. Often, radiographs will be inconclusive in the setting of abdominal pain, and further imaging evaluation will be necessary. Because abdomen radiographs can be the first imaging study for a patient with abdominal pain, timely and accurate interpretation can be critical to patient management and outcome.




Fluoroscopy



Clinical Utility


The use of fluoroscopy with various intraluminal contrast agents to diagnose abnormalities of the GI tract has decreased markedly in recent years. Together with a decline in usage has come a decline in expertise. The falling number of “barium gurus” will doubtless contribute to a further decline in barium studies. This trend, coupled with the increase of competing modalities such as multidetector CT and endoscopy, has led some radiologists to conclude that barium serves no purpose in today’s modern radiology department. In some respects, this is a self-fulfilling prophecy, because individuals scantily trained to perform high-quality examinations are likely to provide little diagnostically useful information during fluoroscopy.


In reality, fluoroscopy plays a limited, but nonetheless important, role in GI problem solving (Box 16-2). As with other imaging modalities, such as US, the problem-solving yield of fluoroscopy can be improved by taking an interactive approach. The timely acquisition of spot films, attention to motility, appropriate patient positioning, and application of compression can markedly improve the diagnostic yield of fluoroscopic examinations. The appropriate usage of specialized techniques such as peroral pneumocolon and enteroclysis can further increase the diagnostic yield.



There are some scenarios in which fluoroscopy can be potentially harmful. These include toxic megacolon and typhlitis. Water-soluble contrast is preferable to barium in cases of suspected bowel perforation, obstipation, chronic intestinal pseudoobstruction, fistula evaluation, and before abdominal surgery or endoscopy.



Fluoroscopic Findings


Box 16-3 and Table 16-1 list key findings visible with GI fluoroscopy. Of course, one must also remember to inspect regions and findings beyond the confines of the GI tract, such as abnormal gas collections, calcifications and other abnormal opacities, organ outlines, and skeletal abnormalities.



Table 16-1 Fluoroscopic Findings and Their Significance

































Finding Possible Significance
Thickened folds (single or double contrast) Inflammation, infection, neoplastic infiltration, intramural hemorrhage, or submucosal edema
Radiating folds (single or double contrast) Normal gastric cardia or pylorus, gastric or duodenal ulcer or ulcer scar
Linear density (double contrast) Linear ulcer, edge of a protruding lesion, edge of an extrinsic lesion, extrinsic structure viewed through bowel, or coaptation artifact
Ring shadow (double contrast) Gas-filled diverticulum, polyp surrounded by gas, gas-filled ulcer, gas bubble, or extrinsic structure viewed through bowel (e.g., pedicle)
Double ring shadow (double contrast) (Fig. 16-11) Polyp on a stalk viewed end-on creates a double ring appearance; this has been referred to as the “Mexican hat sign”; do not confuse this with a barium droplet hanging from sessile polyp or barium pooled in an ulcer crater or diverticulum; in these latter cases, the center of the inner ring will be uniformly dense rather than lucent
Bowler hat (double contrast) (Fig. 16-12) Sessile polyp (dome of hat points to lumen) or diverticulum (dome of hat points away from lumen)
Focal round density (double contrast) Stalactite phenomenon (barium dripping from a protruding lesion of the nondependent surface), ulcer, diverticulum, barium precipitate, barium trapped in mucosal fold or pit, or extrinsic density (e.g., calcification)
Filling defect (single contrast) (Fig. 16-13) Mucosal or submucosal mass, food, stool, foreign body, bubble, blood clot, redundant or nodular mucosa
Bull’s-eye/target lesion (single or double contrast) Hematogenous metastasis, ulcerated submucosal tumor, aphthous lesion, or ectopic pancreatic rest





Ultrasound


In the United States, ultrasound is rarely used as a stand-alone technique for assessment of bowel. In most general radiology practices, it is more likely that a bowel abnormality will be discovered incidentally during an abdominal sonographic survey than as the result of a directed search (Fig. 16-14). It is perhaps most important that one be able to distinguish between normal and abnormal bowel with US, because additional diagnostic tests will likely be necessary before a definitive diagnosis can be established.



Normal bowel is compressible by the US transducer, and peristalsis can be observed during real-time imaging. Water administered by mouth or per rectum aids in distinguishing bowel from other structures or conditions such as abscess. Normal bowel typically demonstrates some degree of stratification with US, and interruption of any of the bowel wall layers may be a sign of abnormality such as tumor. A thick hypoechoic ring (representing thickened bowel wall) surrounding a hyperechoic residual lumen, ulcerated mucosa, or both is referred to as the “pseudokidney sign.” This appearance is reasonably predictive of bowel pathology.


When bowel wall thickening is present, one should attempt to distinguish between short-segment thickening (more worrisome for a malignant process) and longsegment thickening (more likely with benign processes such as inflammation or infection). Bowel wall tumors are usually hypoechoic. Increased echogenicity in the fat adjacent to bowel may be a sign of inflammation. Gas trapped in ulcerated mucosa produces linear echogenic foci often associated with ring-down artifact. Pneumatosis produces echogenic foci in the bowel wall that can shadow. The normal bowel wall shows minimal signal with color Doppler. Completely absent flow with color Doppler can be a sign of intestinal ischemia, whereas increased flow suggests inflammation or neoplasia.




Pelvic bowel loops that are difficult to evaluate transabdominally may be better visualized with transvaginal US, although transvaginal US is rarely performed for this indication. Endorectal US is commonly performed for the staging of rectal carcinoma. Although endorectal US cannot assess distant disease, it is a relatively accurate method of determining T stage of rectal cancer. Loss or discontinuity of the normal layered appearance of the rectal wall implies tumor invasion. Endoscopic US is also useful for the diagnosis and staging of gastric tumors, and it has the advantage of facilitating biopsy of any abnormalities encountered.




Magnetic Resonance Imaging



Indications for Magnetic Resonance Imaging of the Bowel


MRI of the bowel has a few significant advantages over CT that may increase utilization for evaluation of the GI tract. These include improved contrast conspicuity and lack of ionizing radiation. However, MRI examinations take significantly longer than the typical CT scan to perform and require greater patient cooperation. As scan times continue to shorten and expertise continues to grow, MRI will doubtless play a larger role in evaluating the GI tract in the near future.


Sequences useful for imaging bowel include single-shot echo train spin-echo sequences, balanced steady-state free precession sequences, and two- or threedimensional gadolinium-enhanced fat-suppressed gradient-echo sequences. Administration of an antiperistaltic agent immediately before contrast-enhanced imaging improves image quality. When performing gadolinium-enhanced imaging to evaluate the bowel wall, a negative contrast agent (dark on T1) that distends the bowel lumen is helpful. A variety of intraluminal contrast agents have been recommended for this purpose, including water, dilute barium, and polyethylene glycol. Additives such as mannitol, Psyllium husk, and locust bean gum have been used to improve bowel distention. Although expertly performed MRI of a cooperative patient is equivalent to CT for evaluation of many suspected GI abnormalities, there are a few indications for which MRI may be preferable to CT (Box 16-5).






ABNORMALITIES OF THE STOMACH



Thickening


Deciding whether the gastric wall is too thick on an imaging study is a challenge, because the gastric wall varies in thickness between its nondistended and distended states. If the study is performed specifically to evaluate the stomach, every reasonable effort should be made to completely distend the lumen by using effervescent granules, water, or other oral contrast material. When only partial distention of the stomach can be achieved, changing the patient’s position can redistribute the gastric contents to distend individual gastric segments. The gastric wall between rugal folds generally measures 5 mm or less. The gastric cardia looks thickened and masslike when not adequately distended, mimicking a carcinoma on CT. The distal antrum and pyloric channel are also common locations of spurious thickening on CT and MRI. Giving gas crystals and scanning the patient left-side-down can help to differentiate real from artifactual thickening. As a general rule, the prepyloric antral wall of the stomach should measure less than 5 mm. The antrum should be considered abnormal when this thickness exceeds 10 mm or if asymmetric thickness is present.


Normally the gastric wall is thinner in nondependent segments of the stomach that are well distended by gas. In addition, adherent debris that may add to the appearance of gastric wall thickening typically remains within the fluid-filled dependent portions. When wall thickening involves not only dependent, fluid-filled parts of the stomach but also nondependent, gas-filled portions on a CT or MRI scan, an underlying abnormality should be suspected (Fig. 16-18).





Thickening of the rugal folds has an extensive differential diagnosis (Table 16-3). Fortunately, many of the entities that cause rugal fold thickening are extremely rare, and many of the common causes will have other imaging findings that allow a specific diagnosis to be made.


Table 16-3 Causes of Rugal Fold Thickening











































Causes Helpful Distinguishers
Common Causes
Underdistention

Gastritis (most often Helicobacter pylori)

Pancreatitis


Portal hypertension


Varices


Lymphoma (Figs. 16-19 and 16-20)

Rare Causes
Zollinger–Ellison syndrome


Ménétrier disease
Eosinophilic gastritis

Amyloidosis
Sarcoidosis



CT, Computed tomography; MR, magnetic resonance; UGI, upper gastrointestinal.




Gastric adenocarcinoma can cause focal or diffuse gastric wall thickening, although the normal rugal fold pattern is typically not maintained at the site of tumor. Scirrhous carcinoma infiltrates the submucosa of the stomach, resulting in some combination of wall thickening, luminal narrowing, and reduced distensibility. Gastric lymphoma can have a variety of appearances including diffuse gastric wall thickening (see Fig. 16-18).





Ulceration


With the widespread availability of endoscopy and development of effective methods of treating peptic ulcer disease, gastric ulcers are rarely encountered by radiologists. Many radiologists will have never diagnosed a gastric ulcer on an upper GI examination during their training.


Most gastric ulcers are found along the posterior wall of the antrum or body, or along the lesser curvature of the stomach. Anterior wall or greater curvature ulcers account for about 15% of gastric ulcers. Of course, this statistic is not particularly useful to the radiologist faced with one particular ulcer on a fluoroscopic study. It is far more useful to know whether to recommend endoscopy for further evaluation when encountering a gastric ulcer. In general, endoscopy should be pursued if any ulcer is less than unequivocally benign in appearance (Table 16-5). Although the other signs are useful from a statistical standpoint, only the rarely seen Hampton line is believed to be an unequivocal sign of benignity.


Table 16-5 Features Suggestive of Benign versus Malignant Gastric Ulcers



































Feature Benign Malignant
Location Lesser curvature and distal stomach more common Greater curvature and proximal stomach more common
Margins Sharp, round Irregular, elevated
Folds Smooth, extend to edge of crater Nodular, amputated, do not extend to crater edge
Mucosa Area gastricae intact Area gastricae distorted or obliterated
Profile Projects beyond expected lumen contour Projects within expected lumen contour
Signs Hampton line* Carman meniscus sign
Clinical course Heals with appropriate peptic ulcer therapy Persists or progresses despite appropriate therapy for peptic ulcer disease

* Hampton line refers to a thin, straight lucent line across the neck of a contrastfilled ulcer representing intact mucosa.


Carman meniscus sign is created when the heaped-up edges of a large shallow malignant ulcer are apposed during compression, trapping barium in a collection that is convex to the gastric lumen.


Erosions are small, superficial linear or round ulcerations that often occur on the crests of gastric folds. Erosions are typically multiple and often demonstrate a radiolucent halo of edematous mucosa on double-contrast barium examinations. Box 16-6 lists some causes of gastric erosions. Note that many of the entities that cause erosions also can result in gastric wall thickening and narrowing when advanced.




Masses



Pseudotumors


The stomach is a common site for a false-positive interpretation regarding a mass. Some common locations for gastric pseudotumors include the region of the gastric cardia, along the proximal lesser curvature, and in the distal antrum and pyloric region. Most often, the appearance of a mass is related to redundant, contracted, or nondistended gastric wall, and the issue can be satisfactorily resolved with further distention of the stomach. Ingested material or bezoar may also give the false impression of a gastric mass (Fig. 16-21). A pseudotumor can change dramatically in position, often does not have a consistent connection with the gastric wall, and may fail to demonstrate enhancement after intravenous contrast administration on CT and MR images. A true gastric neoplasm is a persistent finding on multiple phases, acquisitions, or examinations that has a consistent communication with the gastric wall and demonstrates enhancement. Movement alone is not a helpful sign because pedunculated gastric masses can change position. Unlike some bezoars, gastric neoplasms do not contain gas. A pancreatic rest can simulate an ulcerated submucosal mass. Ectopic pancreatic tissue usually occurs along the greater curvature of the distal gastric antrum or in the proximal duodenum. Pancreatic rests are usually small (1 cm or less), broad based, smooth, and have a central umbilication related to the primitive ductal system as seen on barium studies.








Common Malignant Gastric Tumors



GASTROINTESTINAL STROMAL TUMOR


GISTs are histologically distinct from leiomyomas, although this distinction cannot be made with imaging. Likewise, the distinction between benign or malignant is difficult unless metastatic disease or frank invasion is present. Most GISTs are sporadic, but these tumors can be associated with neurofibromatosis (NF-1) and Carney triad (gastric GIST, pulmonary chondroma, extraadrenal paraganglioma). GISTs are usually solitary, often large, round tumors of the gastric wall. They can appear primarily exophytic (common), intramural, or endophytic (i.e., bulging into gastric lumen [least common]). GISTs are often necrotic, demonstrating mucosal ulceration or deep cavitation that can communicate with the gastric lumen. Up to 25% of lesions demonstrate calcifications on CT. GISTs usually are of intermediate signal intensity on T1-weighted MRI and intermediate to low signal intensity on T2-weighted MRI. Areas of central necrosis demonstrate increased signal intensity on T2-weighted images. Enhancement after intravenous contrast administration is variable on CT and MRI (necrotic areas do not enhance). GISTs tend to be 18F-FDG avid. Therefore, PET is a useful modality for documenting early response to imatinib. Malignant tumors can spread to liver, peritoneum, and rarely, lymph nodes. Metastatic foci often resemble the primary tumor, although metastases treated with imatinib can mimic cysts.










Mar 6, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on Gastrointestinal Tract
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